The ubiquitin conjugation system (UCS) plays a major role in the degradation of cellular proteins in eukaroytic cells and in some bacterial cells. UCS mediates the elimination of abnormal proteins and regulates the half-lives of other important regulatory proteins that control gene transcription and cell cycle progression. The UCS is reported to degrade mitotic cyclic kinases, oncoproteins, tumor suppressors, viral proteins, transcriptional regulators, and receptors associated with signal transduction (Verma, R. et al. (1997) Science 278:455-460; Ciechanover, A. (1994) Cell 79:13-21).
Several steps are involved in ubiquitin (Ub) conjugation and protein degradation (Jentsch, S. (1992) Annu. Rev. Genet. 26:179-207). First, Ub, a small, heat stable protein, is activated by a ubiquitin-activating enzyme (E1). This ATP dependent activation involves binding of the C-terminus of Ub to the thiol group of a cysteine residue of E1. Activated Ub is subsequently transferred to one of several Ub-conjugating enzymes (E2). Each E2 has a recognition subunit which allows it to interact with proteins carrying a particular degradation signal. E2 links the Ub molecule through its C-terminal glycine to an internal lysine of the target protein. Different ubiquitin-dependent proteolytic pathways employ structurally similar, but distinct, E2s, and in some instances, accessory factors known as ubiquitin-ligases or E3s, are required to work in conjunction with E2s for recognition of certain substrates (Jensen, J. P. et al. (1995) J. Biol. Chem. 270:30408-30414). More than one Ub molecule may be needed to ubiquinate a target protein which is subsequently recognized and degraded by a proteasome. After degradation, Ub is released and reutilized.
Prior to activation, Ub is usually expressed as a fusion protein composed of an N-terminal ubiquitin and a C-terminal extension protein (CEP) or as a polyubiquitin protein with Ub monomers attached head to tail. CEPs have characteristics of a variety of regulatory proteins; most are highly basic, contain up to 30% lysine and arginine residues, and have nucleic acid-binding domains (Monia, B. P. et al. (1989) J. Biol. Chem. 264:4093-4103). The fusion protein is an important intermediate which appears to mediate co-regulation of the cell's translational and protein degradation activities, as well as localization of the inactive enzyme to specific cellular sites. Once delivered, C-terminal hydrolases cleave the fusion protein to release a functional Ub (Monia et al., supra).
The E2s are important for substrate specificity in several UCS pathways. All E2s have a conserved ubiquitin conjugation (UBC) domain of approximately 16 kD, at least 35% identity with each other, and contain a centrally located cysteine residue which is necessary for ubiquitin-enzyme thiolester formation (Jentsch, supra). A highly conserved proline-rich element is located N-terminal to the active cysteine residue. Structural variations outside of this conserved domain are used to separate the E2 enzymes into classes. The E2s of class 1 (E2-1) consist of the conserved UBC domain and include yeast E2-1 and UBCs 4, 5, and 7. These E2s are thought to require E3 to carry out their activities (Jentsch, supra). UBC7 has been shown to recognize ubiquitin as a substrate and to form polyubiquitin chains in vitro (van Nocker, S. et al. (1996) J. Biol. Chem. 271:12150-58). E2s of class 2 (E2-2) have various unrelated C-terminal extensions that contribute to substrate specificity and cellular localization. The yeast E2-2 enzymes, UBC2 and UBC3, have highly acidic C-terminal extensions that promote interactions with basic substrates such as histones. Yeast UBC6 has a hydrophobic signal-anchor sequence that localizes the protein to the endoplasmic reticulum.
Abnormal activities of the UCS are implicated in a number of diseases and disorders. These include, e.g., cachexia (Llovera, M. et al. (1995) Int. J. Cancer 61: 138-141), degradation of the tumor-suppressor protein, p53 (Ciechanover, supra), and neurodegeneration such as observed in Alzheimer's disease (Gregori, L. et al. (1994) Biochem. Biophys. Res. Commun. 203: 1731-1738). Since ubiquitin conjugation is a rate-limiting step in antigen presentation, the ubiquitin degradation pathway may also have a critical role in the immune response (Grant E. P. et al. (1995) J. Immunol. 155: 3750-3758).
The discovery of new ubiquitin-conjugating-like protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cancer, and developmental, immune and neuronal disorders.